iOS多线程之NSOperation

NSOperation简介

NSOperation是苹果提供给我们的一套多线程解决方案。实际上NSOperation是基于GCD更高一层的封装，但是比GCD更简单易用、代码可读性也更高。

1. NSOperation及其子类

NSOperation是个抽象类，并不能封装任务。我们只有使用它的子类来封装任务。

NSInvocationOperation
NSBlockOperation
自定义NSOperation

1.1 NSInvocationOperation

- (void)invocationOperationTest {
    NSLog(@"InvocationOperationCurrentThread:%@", [NSThread currentThread]); // 打印当前线程
    NSInvocationOperation *operation = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"InvocationOperation"}];
    [operation start];
}

- (void)taskTest:(NSDictionary *)dict {
    NSString *name = [dict valueForKey:@"name"];
    for (int i = 0; i < 2; i++) {
        [NSThread sleepForTimeInterval:2]; // 模拟耗时操作
        NSLog(@"%@---%@", name,[NSThread currentThread]); // 打印当前线程
    }
}
//输出结果
2018-05-24 17:14:46.893 MultiThreadingDemo[3979:594628] InvocationOperationCurrentThread:{number = 1, name = main}
2018-05-24 17:14:48.895 MultiThreadingDemo[3979:594628] InvocationOperation---{number = 1, name = main}
2018-05-24 17:14:50.897 MultiThreadingDemo[3979:594628] InvocationOperation---{number = 1, name = main}

从结果来看，NSInvocationOperation是在主线程中执行的，并没有开辟新线程，那是不是说明NSInvocationOperation一定在主线程执行呢？
show the code测试一下

- (void)invocationInOtherThread {
    NSThread *thread1 = [[NSThread alloc] initWithTarget:self selector:@selector(invocationOperationTest) object:nil];
    thread1.name=@"invocationThread";
    [thread1 start];
}
//输出结果
2018-05-24 17:23:33.505 MultiThreadingDemo[3979:608510] InvocationOperationCurrentThread:{number = 3, name = invocationThread}
2018-05-24 17:23:35.511 MultiThreadingDemo[3979:608510] InvocationOperation---{number = 3, name = invocationThread}
2018-05-24 17:23:37.514 MultiThreadingDemo[3979:608510] InvocationOperation---{number = 3, name = invocationThread}

结果证明我们之前的推测是错误的，NSInvocationOperation在我们新开辟的线程中执行。

结论：单独使用子类 NSInvocationOperation 执行一个操作的情况下，操作是在当前线程（非主线程）执行的，并没有开启新线程。

1.2 NSBlockOperation

首先看下API提供的方法：

//类方法
+ (instancetype)blockOperationWithBlock:(void (^)(void))block;
//实例方法
- (void)addExecutionBlock:(void (^)(void))block;
//封装的任务
@property (readonly, copy) NSArray *executionBlocks;

上代码

- (void)blockOperationTest {
    NSLog(@"BlockOperationCurrentThread:%@", [NSThread currentThread]); // 打印当前线程
    NSBlockOperation *operation = [NSBlockOperation blockOperationWithBlock:^{
        [self taskTest:@{@"name":@"BlockOperation1"}];
    }];
    [operation addExecutionBlock:^{
        [self taskTest:@{@"name":@"BlockOperation2"}];
    }];
    [operation addExecutionBlock:^{
        [self taskTest:@{@"name":@"BlockOperation3"}];
    }];
    [operation start];
}
//输出结果
2018-05-24 17:29:15.550 MultiThreadingDemo[4000:617102] BlockOperationCurrentThread:{number = 1, name = main}
2018-05-24 17:29:17.553 MultiThreadingDemo[4000:617102] BlockOperation1---{number = 1, name = main}
2018-05-24 17:29:17.580 MultiThreadingDemo[4000:617158] BlockOperation2---{number = 3, name = (null)}
2018-05-24 17:29:17.580 MultiThreadingDemo[4000:617157] BlockOperation3---{number = 4, name = (null)}
2018-05-24 17:29:19.554 MultiThreadingDemo[4000:617102] BlockOperation1---{number = 1, name = main}
2018-05-24 17:29:19.655 MultiThreadingDemo[4000:617158] BlockOperation2---{number = 3, name = (null)}
2018-05-24 17:29:19.655 MultiThreadingDemo[4000:617157] BlockOperation3---{number = 4, name = (null)}

结论：和NSInvocationOperation相比，NSBlockOperation对象不用添加到操作队列也能开启新线程，但是开启新线程是有条件的。前提是一个blockOperation中需要封装多个任务。如果只开启一个任务，默认会在当前线程执行。

NSOperation是基于GCD更高一层的封装，同样拥有任务的概念，那么我们可不可以简单的认为NSInvocationOperation类似于dispatch_sync，NSBlockOperation类似于dispatch_async呢？个人意见，欢迎指正。

1.3 自定义NSOperation子类

稍后更新...

2. NSOperationQueue

队列，回想GCD里面的dispatch_queue_t中...
切入正题：

NSOperation 需要配合 NSOperationQueue 来实现多线程。因为默认情况下，NSOperation 单独使用时系统同步执行操作，配合 NSOperationQueue 我们能更好的实现异步执行。（copy自https://www.jianshu.com/p/4b1d77054b35）

2.1 NSOperationQueue创建

NSOperation 实现多线程的使用步骤分为三步：

创建操作：先将需要执行的操作封装到一个 NSOperation 对象中。

NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"Invocation1InQueue"}];
NSInvocationOperation *operation2 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"Invocation2InQueue"}];

创建队列：创建 NSOperationQueue 对象。

NSOperationQueue *queue = [[NSOperationQueue alloc] init];

将操作加入到队列中：将 NSOperation 对象添加到 NSOperationQueue 对象中。

[queue addOperation:operation1];
[queue addOperation:operation2];

2.2 NSOperationQueue中的依赖

NSOperation、NSOperationQueue 最吸引人的地方是它能添加操作之间的依赖关系。通过操作依赖，我们可以很方便的控制操作之间的执行先后顺序。

看下api中关于dependcy的几个方法
addDependency:添加依赖
removeDependency:移除依赖
NSArray *dependencies在当前操作开始执行之前完成执行的所有操作对象数组。比如A依赖B，B依赖C，那么A的dependencies就是BC

- (void)dependencyQueueTest {
    //依赖在开发中还是经常使用到的：有 A、B 两个操作，其中 A 执行完操作，B 才能执行操作
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    //添加InvocationOperation
    NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"dependency1InQueue"}];
    NSInvocationOperation *operation2 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"dependency2InQueue"}];
    //添加BlockOperation
    NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{
        [self taskTest:@{@"name":@"dependency3InQueue"}];
    }];
    [operation1 addDependency:operation3];//operation1依赖operation3
    [operation2 addDependency:operation1];//operation2依赖operation1
    [queue addOperation:operation1];
    [queue addOperation:operation2];
    [queue addOperation:operation3];
}
//输出结果：
2018-05-24 18:11:48.881 MultiThreadingDemo[4000:656437] dependency3InQueue---{number = 5, name = (null)}
2018-05-24 18:11:50.945 MultiThreadingDemo[4000:656437] dependency3InQueue---{number = 5, name = (null)}
2018-05-24 18:11:53.012 MultiThreadingDemo[4000:656437] dependency1InQueue---{number = 5, name = (null)}
2018-05-24 18:11:55.085 MultiThreadingDemo[4000:656437] dependency1InQueue---{number = 5, name = (null)}
2018-05-24 18:11:57.114 MultiThreadingDemo[4000:680839] dependency2InQueue---{number = 15, name = (null)}
2018-05-24 18:11:59.147 MultiThreadingDemo[4000:680839] dependency2InQueue---{number = 15, name = (null)}

优先级NSOperationQueuePriority

typedef NS_ENUM(NSInteger, NSOperationQueuePriority) {
    NSOperationQueuePriorityVeryLow = -8L,//最低
    NSOperationQueuePriorityLow = -4L,
    NSOperationQueuePriorityNormal = 0,//默认
    NSOperationQueuePriorityHigh = 4,
    NSOperationQueuePriorityVeryHigh = 8//最高
};

2.3 优先级queuePriority

queuePriority是NSOperation的属性，并不是Queue的，是operation在queue中的优先级

- (void)queuePriorityTest {
    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    //添加operation1，设置operation1的优先级为VeryLow
    NSInvocationOperation *operation1 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"PriorityVeryLow"}];
    operation1.queuePriority = NSOperationQueuePriorityVeryLow;
    //添加operation2，默认优先级为normal
    NSInvocationOperation *operation2 = [[NSInvocationOperation alloc] initWithTarget:self selector:@selector(taskTest:) object:@{@"name":@"PriorityNormal"}];
    //添加operation3，优先级为VeryHigh
    NSBlockOperation *operation3 = [NSBlockOperation blockOperationWithBlock:^{
        [self taskTest:@{@"name":@"PriorityVeryHigh"}];
    }];
    operation3.queuePriority = NSOperationQueuePriorityVeryHigh;
    [queue addOperation:operation1];
    [queue addOperation:operation2];
    [queue addOperation:operation3];
}
//输出结果
2018-05-24 18:17:55.987 MultiThreadingDemo[4000:656437] PriorityVeryLow---{number = 5, name = (null)}
2018-05-24 18:17:55.987 MultiThreadingDemo[4000:690639] PriorityNormal---{number = 16, name = (null)}
2018-05-24 18:17:55.987 MultiThreadingDemo[4000:690641] PriorityVeryHigh---{number = 17, name = (null)}
2018-05-24 18:17:57.996 MultiThreadingDemo[4000:656437] PriorityVeryLow---{number = 5, name = (null)}
2018-05-24 18:17:57.996 MultiThreadingDemo[4000:690639] PriorityNormal---{number = 16, name = (null)}
2018-05-24 18:17:57.996 MultiThreadingDemo[4000:690641] PriorityVeryHigh---{number = 17, name = (null)}

从结果看：queuePriority跟依赖是不一样的，并不是优先级高的先执行完再执行优先级低的。

2.4 最大并发操作数：maxConcurrentOperationCount

maxConcurrentOperationCount 默认情况下为-1，表示不进行限制，可进行并发执行。
maxConcurrentOperationCount 为1时，队列为串行队列。只能串行执行。
maxConcurrentOperationCount 大于1时，队列为并发队列。

这个比较简单，就先不上code了
可以对比一下GCD的信号量semaphore

2.5 线程之间通信

最常用的：异步操作完成之后回主线程更新UI

NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperationWithBlock:^{
    [NSThread sleepForTimeInterval:0.5]; // 模拟耗时操作
    // 执行完当前任务刷新UI
    [[NSOperationQueue mainQueue] addOperationWithBlock:^{
        NSLog(@"task1 have done");
    }];
}];

2.6 线程锁

未完待续...

参考链接：(排名不分先后)
https://www.jianshu.com/p/4b1d77054b35
https://www.jianshu.com/p/e6cd25f6da91
https://www.jianshu.com/p/6861d36b1c8e